In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The photolithography which is currently on widespread use in the art is approaching the essential limit of resolution determined by the wavelength of a light source. As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp was widely used in 1980's. Reducing the wavelength of exposure light was believed effective as the means for further reducing the feature size. For the mass production process of 64 MB dynamic random access memories (DRAM, processing feature size 0.25 μm or less) in 1990's and later ones, the exposure light source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm.
However, for the fabrication of DRAM with a degree of integration of 256 MB and 1 GB or more requiring a finer patterning technology (processing feature size 0.2 μm or less), a shorter wavelength light source was required. Over a decade, photolithography using ArF excimer laser light (193 nm) has been under active investigation. It was expected at the initial that the ArF lithography would be applied to the fabrication of 180-nm node devices. However, the KrF excimer lithography survived to the mass-scale fabrication of 130-nm node devices. So, the full application of ArF lithography started from the 90-nm node. The ArF lithography combined with a lens having an increased numerical aperture (NA) of 0.9 is considered to comply with 65-nm node devices. For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the development of F2 lithography was stopped and instead, the ArF immersion lithography was introduced.
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water having a refractive index of 1.44. The partial fill system is compliant with high-speed scanning and when combined with a lens having a NA of 1.3, enables mass production of 45-nm node devices.
One candidate for the 32-nm node lithography is lithography using extreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUV lithography has many accumulative problems to be overcome, including increased laser output, increased sensitivity, increased resolution and minimized edge roughness (LER, LWR) of resist film, defect-free MoSi laminate mask, reduced aberration of reflection mirror, and the like.
Another candidate for the 32-nm node lithography is high refractive index liquid immersion lithography. The development of this technology was stopped because LuAG, a high refractive index lens candidate had a low transmittance and the refractive index of liquid did not reach the goal of 1.8.
Under the circumstances, measures for prolonging the ArF immersion lithography were sought. One candidate measure is double patterning. Typical of the double patterning are the self-aligned double patterning (SADP) process of adding film to opposite side walls of lines of a resist pattern and the litho-etch-litho-etch (LELE) process of processing a substrate on every patterning. The SADP process is applied to simple line patterns whereas the LELE process is applied to complex patterns and hole patterns. A combination of SADP with LELE is also contemplated.
Recently a highlight is put on the organic solvent development. A very fine hole pattern, which is not achievable with the positive tone, is resolvable through negative tone development with organic solvent. As the ArF resist composition for negative tone development with organic solvent, positive ArF resist compositions of the prior art design may be used. Such pattern forming processes are described in Patent Documents 1 to 3.
In an attempt to form a line pattern of negative tone by organic solvent development, pattern collapse is a problem. When a line-and-space pattern of positive tone is formed via alkaline development, pattern collapse can occur if the pattern has a high aspect ratio. In contrast, the line pattern of negative tone takes an inversely tapered profile due to absorption of the resist, so that greater stresses are applied to the pattern top during spin drying after rinsing. For this reason, the negative tone pattern is more likely to collapse. It is desired to have a negative tone pattern forming process capable of preventing pattern collapse and a material used therein.